The explosive growth of cellular radio during the late 1980s and early 1990s highlighted the importance of mobile telephone communications. Almost all large world wide cities in the developed world now enjoy some form of cellular communications. However, there are still some areas that are not served by cellular. In some developing nations there is not enough of a demand to install wide area cellular systems. In developed nations, there are still remote areas that have not yet been covered by the cellular system.
One approach that has enjoyed considerable press is the idea of creating a satellite system that would provide telephone service the world over. The argument for such an approach is that it would provide coverage to developing countries, and also provide coverage in remote areas in developed countries. (In addition many land line phone companies have less than stellar reputations in some countries, and it is assumed that many citizens would also use a newer and more reliable service.)
In addition, the cost of cellular radio is quite high, and there is a good possibility that satellite systems would yield a lower cost to the final user. Some of this cost savings will be from a simplified business structure. Today, a cellular user might begin his call with a local cellular operator that switches his call to the local bell telephone company that further processes and switches his call. Perhaps a single satellite system could route calls to the final user with fewer different business involvements that would lead to a lower cost.
The physics of satellite orbits, and the physics of radio propagation are very important in the design of a satellite radio telephone system. Radio propagation is such that the available higher frequencies for such a system will not bend around the curvature of the earth, and that such systems will be generally limited to line of sight communications.
The first approach that would normally be considered is to place a geostationary satellite in orbit high above the earth. The first satellite radio links between the U.S.A. and Europe used this approach. There are several weaknesses to this approach:
1) The very high altitude of geostationary satellites causes limited frequency reuse in the system. Since spectrum is a valuable resource, such high altitude satellites are very spectrally inefficient in that the channels could only be used once in a very large area. PA1 2) The very high altitude of geostationary satellites implies that there is a considerable distance between the phone user and his satellite. This large distance requires large transmitter power and/or a large antenna system at both ends of the communication link. These two limitations are especially difficult for the roaming mobile telephone user.
A second more practical approach is to place a pattern of orbiting satellites into lower earth orbits. These multiple satellites would permit some frequency reuse, and these lower altitude satellites would be closer to the earth surface, and thus smaller antennas and less power would be required. This approach of low earth orbit satellites has received considerable press, and they are frequently referred to as LEOS.
Several consortiums of large companies have proposed various approaches built around LEOS. The strategy would be to use the cellular radio concept of not using the same channel on adjacent satellites, and yet to create a grid of orbits such that coverage of the earth would be guaranteed. One proposed grid, however, would have orbits that rotate equally north and south of the equator and thus avoid the poles and simultaneously enhance coverage in the regions where the people reside. The Motorola Iridium concept, however, has orbits that rotate in a longitudinal manner around the earth.
U.S. Pat. No. 5,274,840 issued to Motorola shows a grid of 48 different satellites in 6 different planes. There are 8 satellites in each plane. The planes all intersect near the axis of rotation of the earth. All 48 satellites orbit over both poles. U.S. Pat. No. 5,161,248 issued to Motorola shows a grid of 77 satellites in similar polar orbits. However, in this system there are 7 planes with 11 satellites in each plane.
U.S. Pat. No. 5,119,504 issued to Motorola explains an anticipated handoff system. Since the satellites know where they are located, and the satellites also know the location of the earth user, they can calculate based on knowledge of their own orbits when handoffs are required.
Although these approaches are technically sound, the cost to create one of these systems is estimated to be near four billion dollars. In fact one recent announcement was budgeted at about 8 billion dollars. Some of the various factors that are contributing to the cost of such as system are as follows:
P1 Approximately 80 satellites are utilized in the present system. Each of these satellites weighs approximately 1,500 pounds. This results in a very expensive satellite, and also in a very expensive rocket launch cost. A total of 105,000 pounds of very high technology equipment has to be put into orbit.
In addition to the expensive and heavy satellites there is a very expensive satellite orbit control system to keep the satellites in their proper orbits. Each satellite needs various rockets, rocket fuel, and orbit control computer/radio technology to keep the satellite in a proper orbit. In case one of the satellites would accidentally end up in an improper orbit, there is a very comprehensive ground computer control system to insure that all the satellites stay in correct orbits, and to disable a satellite when it is not in the proper grid orbit. In addition, the complete failure of any one satellite would require the moving of a back up unit to take its place in the grid.